Watch for Muddy Storm Water in Your Neighborhood

During the last big rainstorm, researcher Alan Heyvaert took the time to show us the difference between stream flow from a natural area and urban runoff from roads. The visual impact was astonishing. Water from the natural stream was clear and had very few suspended particles. On the other hand, urban runoff from a nearby road was dark brown and cloudy.
After several hours, the urban runoff samples slowly got lighter in color and slightly less cloudy, but the water never returned to clear. This also happens in Lake Tahoe. Whenever you see murky or brownish water flowing downslope during a storm, that water is going to cause clarity problems for the lake.
Heyvaert likened the roads around Lake Tahoe to a mortar and pestle. “Cars driving around the Tahoe Basin grind the soils and winter traction sands on roadways into a fine dust… like a mortar and pestle.” This pulverized material then is washed by storm water and blown by wind into surrounding areas, and ultimately into the lake. In addition, any other soil erosion that occurs around the watershed tends to mobilize particles of all sizes, which travel toward the lake. It is the smallest of these particles that are the most damaging to water clarity. They remain suspended in the lake water for a long time because they are too small to settle rapidly to the bottom.
Too small to see
The average human hair has a diameter of about 100 microns, and it takes about 2,500 microns to make an inch. For Lake Tahoe, it is the sediment particles ranging from about 1 micron to 10 microns that are of particular concern. These particles are too small to see except with a powerful microscope, but due to their relative abundance in the water column and their optical properties, they are the largest contributing factor to Tahoe’s clarity loss.
Theoretical calculations show that a 10-micron soil particle of typical shape and density will take over 100 days to settle to the average depth of the lake floor (313 meters or 1,027 feet) under ideal conditions (without wind-driven currents or lake upwellings). A 4-micron particle will take almost 1,000 days to settle to this depth under ideal conditions. However, storm winds usually mix the lake water to a depth of 300 meters or more each winter, so many of these particles are brought back up to the surface before they can reach the bottom. Particles sizes of 1 micron to 10 microns are extremely effective at scattering light, which is a major cause of clarity loss. Researchers now contend that approximately 50 percent of the light scattering that represents clarity loss in Lake Tahoe is caused by small, inorganic sediment particles. About 30 percent of the clarity loss is due to algae, and another 20 percent is due to dissolved organic matter in the water. Although these proportions vary seasonally, it is clear that small, inorganic sediment particles are contributing 50 percent or more of the total clarity loss in the lake.
If resource and property managers at Lake Tahoe want to improve the lake’s clarity, they must retain fine sediments up in the watershed, in the soil-plant community. Erosion must be controlled by implementing best management practices (BMPs). Property owners must infiltrate runoff as near to its source as possible, before it can erode soil. One effective method is to plant native and adapted plants, such as those listed in the “Home Landscaping Guide for Lake Tahoe,” which are effective at infiltrating water and holding soil in place. (Call University of Nevada Cooperative Extension, 832-4150, for information on obtaining a copy of the guide.)
To remove sediment that is suspended in runoff, public jurisdictions often construct storm-water treatment basins that cause the sediment in runoff to settle to the bottom of the basins. Unfortunately, storm-water detention basins are not usually effective at removing particles smaller than about 20 microns in diameter. Data is now being collected to determine if wetland retention basins, designed as biological systems that treat storm water, might be more effective at removing the fine sediments suspended in runoff.
If wetland retention basins are more effective, then more of them will be designed and constructed. This is an example of how the Tahoe Basin is employing adaptive management. Scientists are doing research on the lake, and the jurisdictions then apply new knowledge to make management practices more effective at preventing lake pollution. Ideally, a subsequent assessment of project performance leads to even better design and management strategies.

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During the last big rainstorm, researcher Alan Heyvaert took the time to show us the difference between stream flow from a natural area and urban runoff from roads. The visual impact was astonishing. Water from the natural stream was clear and had very few suspended particles. On the other hand, urban runoff from a nearby road was dark brown and cloudy.
After several hours, the urban runoff samples slowly got lighter in color and slightly less cloudy, but the water never returned to clear. This also happens in Lake Tahoe. Whenever you see murky or brownish water flowing downslope during a storm, that water is going to cause clarity problems for the lake.
Heyvaert likened the roads around Lake Tahoe to a mortar and pestle. “Cars driving around the Tahoe Basin grind the soils and winter traction sands on roadways into a fine dust… like a mortar and pestle.” This pulverized material then is washed by storm water and blown by wind into surrounding areas, and ultimately into the lake. In addition, any other soil erosion that occurs around the watershed tends to mobilize particles of all sizes, which travel toward the lake. It is the smallest of these particles that are the most damaging to water clarity. They remain suspended in the lake water for a long time because they are too small to settle rapidly to the bottom.
Too small to see
The average human hair has a diameter of about 100 microns, and it takes about 2,500 microns to make an inch. For Lake Tahoe, it is the sediment particles ranging from about 1 micron to 10 microns that are of particular concern. These particles are too small to see except with a powerful microscope, but due to their relative abundance in the water column and their optical properties, they are the largest contributing factor to Tahoe’s clarity loss.
Theoretical calculations show that a 10-micron soil particle of typical shape and density will take over 100 days to settle to the average depth of the lake floor (313 meters or 1,027 feet) under ideal conditions (without wind-driven currents or lake upwellings). A 4-micron particle will take almost 1,000 days to settle to this depth under ideal conditions. However, storm winds usually mix the lake water to a depth of 300 meters or more each winter, so many of these particles are brought back up to the surface before they can reach the bottom. Particles sizes of 1 micron to 10 microns are extremely effective at scattering light, which is a major cause of clarity loss. Researchers now contend that approximately 50 percent of the light scattering that represents clarity loss in Lake Tahoe is caused by small, inorganic sediment particles. About 30 percent of the clarity loss is due to algae, and another 20 percent is due to dissolved organic matter in the water. Although these proportions vary seasonally, it is clear that small, inorganic sediment particles are contributing 50 percent or more of the total clarity loss in the lake.
If resource and property managers at Lake Tahoe want to improve the lake’s clarity, they must retain fine sediments up in the watershed, in the soil-plant community. Erosion must be controlled by implementing best management practices (BMPs). Property owners must infiltrate runoff as near to its source as possible, before it can erode soil. One effective method is to plant native and adapted plants, such as those listed in the “Home Landscaping Guide for Lake Tahoe,” which are effective at infiltrating water and holding soil in place. (Call University of Nevada Cooperative Extension, 832-4150, for information on obtaining a copy of the guide.)
To remove sediment that is suspended in runoff, public jurisdictions often construct storm-water treatment basins that cause the sediment in runoff to settle to the bottom of the basins. Unfortunately, storm-water detention basins are not usually effective at removing particles smaller than about 20 microns in diameter. Data is now being collected to determine if wetland retention basins, designed as biological systems that treat storm water, might be more effective at removing the fine sediments suspended in runoff.
If wetland retention basins are more effective, then more of them will be designed and constructed. This is an example of how the Tahoe Basin is employing adaptive management. Scientists are doing research on the lake, and the jurisdictions then apply new knowledge to make management practices more effective at preventing lake pollution. Ideally, a subsequent assessment of project performance leads to even better design and management strategies.